1. Field of the Invention
The present invention relates to a cathode-ray tube phosphor and, more particularly, to a color cathode-ray tube phosphor suitably used to form a phosphor screen on a faceplate of a cathode-ray tube by an outer surface exposure method.
2. Description of the Related Art
Cathode-ray tube phosphors are generally coated on the inner surface of a faceplate in a cathode-ray tube.
FIG. 5 shows one embodiment of a cathode-ray tube. As is shown in FIG. 5, a vacuum envelope 1 made of glass is arranged such that a faceplate 2 is integrally connected to a funnel 3. The funnel 3 is provided with a neck portion 4. An electron gun 6 for emitting and focusing electron beams 5 is arranged within the neck 4. A phorphor screen 7 is formed on the inner surface of the faceplate 2 of the envelope 1, such that the phosphor screen 7 opposes to the electron gun 6. A shadow mask structure 8 is arranged on the inside of the phosphor screen 7.
As a conventional exposure method of forming a phosphor screen 7 of a so-called black matrix type color cathode-ray tube, a method of exposing a coating layer, which is formed on tinner surface of a faceplate 2 and on which a phosphor slurry containing a photosensitive substance and a phosphor, through a shadow mask 8 by using a light source arranged at the position of an electron gun 6 of a cathode-ray tube is available. In this conventional method, however, when a photopolymerization reaction progresses from the surface of the coating film exposed in accordance with an exposure time and an exposure intensity toward the inner surface of the faceplate, unsatisfactory adhesion strength between the phosphor film and the faceplate inner surface and errors in the shape and position of the phosphor film are caused due to refraction of radiated light, an error in an exposure position, nonuniformity in film thickness and the like. Therefore, it is difficult to form a phosphor film having a predetermine shape, a high resolution, a sharp edge, and a high density in a predetermined position of the faceplate inner surface.
In recent years, however, Published Unexamined Japanese Patent Application No. 60-119055 or Published Examined Japanese Patent Application No. 63-42371 discloses an outer surface exposure method in which a first photosensitive agent layer is selectively formed on a portion of the inner surface of a faceplate except for a portion on which a phosphor layer is to be formed, a phosphor slurry containing a second photosensitive agent which cannot be removed by a reverse developing agent of the first photosensitive layer is coated, exposure is performed from the outer surface of the faceplate, and development is performed by the reverse developing agent of this resist layer after exposure, thereby removing the first photosensitive agent layer and the phosphor slurry at a portion except for the portion on which a phosphor layer is to be formed.
In this outer surface exposure method, a polyvinyl alcohol (PVA)-ammonium dichromate (ADC)-based photosensitive agent and a PVA-stilbazolium (SBQ)-based photosensitive agent are often used as the first and second photosensitive agents, respectively. In addition, hydrogen peroxide is often used as the reverse developing agent of the first photosensitive agent.
According to this method, in a step of forming a phosphor film of one color, reverse development must be performed once by using hydrogen peroxide. Therefore, this development is performed three times in order to form a phosphor screen consisting of phosphor films of three colors. For this reason, a phosphor film formed first is subjected to reverse development twice in two subsequent phosphor film formation steps.
This hydrogen peroxide oxidizes the surfaces of ZnS, Y.sub.2 O.sub.2 S, and the like as phosphor components to produce H.sub.2 SO.sub.4 and H.sub.2 S, thereby reducing brightness of each film.
In addition, hydrogen peroxide causes denaturing of the second photosensitive agent PVA-SBQ to deteriorate its function as a photosensitive agent. For this reason, phenomena such as omission of phosphor dots and stripes occur.
It is found by later studies for further improvements that the second photosensitive agent can be prevented from being deteriorated by mixing a vinyl acetate emulsion as a protective colloid and oxyethylenedodecylamine, for example, as a cationic surfactant in the photosensitive agent. In addition, in order to prevent brightness reduction caused by oxidation of the phosphor surface, Published Unexamined Japanese Patent Application No. 63-207888 discloses a method in which an acrylic resin, an acrylic monomer, polystyrene, or the like is coated on the surface of a phosphor particle to improve an oxidation resistance, and a silica treatment is performed on this coating layer in accordance with a conventional method. Although the obtained phosphor has a satisfactory oxidation resistance, it cannot be satisfactorily dispersed in the cationic surfactant described above. Therefore, a good phosphor screen cannot be formed by using this phosphor.